1. Field of the Invention
The present invention relates to a gallium nitride-based semiconductor light emitting device, and particularly to a gallium nitride-based semiconductor light emitting device which employs Ag in the electrode for increased light reflectivity, while preventing migration of Ag and exhibiting increased reliability as a light emitting diode.
2. Description of the Related Art
Group III nitride semiconductors such as gallium nitride have a direct band gap of energy corresponding from the visible light range to the ultraviolet range and are capable of high efficiency luminosity, and therefore they are used in products such as light emitting diodes (LED) and laser diodes (LD). In particular, the realization of white light emitting diodes by combination with fluorescent materials has widened their potential uses as illuminating LEDs, and the market for these is increasing. In order to obtain an illuminating LED it is first necessary to achieve the high efficiency of a fluorescent lamp (1 m/W).
A common structure for high-efficiency Group III nitride semiconductor elements is a flip-chip structure wherein the electrode surface and luminescent region are mounted on the lower side of a pedestal, and light is extracted from the sapphire base side.
Since in a flip-chip structure the element region is mounted below on a pedestal via a metal electrode, the heat resistance is lower than in a face-up element structure in which it is mounted via the substrate, and the structure is therefore advantageous for uses in which a pulse heavy current is applied for high light emission output.
In the flip-chip structure, light reflected at the electrode constitutes a major proportion of the output, and therefore a metal with high reflectivity must be selected for the electrode metal. Among metals, Ag is known to have high reflectivity in the visible light range, and a flip-chip element employing an Ag reflective electrode can achieve a 10-20% increase in output over a flip-chip element employing a different metal.
However, Ag metal is also known to be prone to electromigration, and electrodes with exposed Ag electrode sections inevitably suffer reduction in light emission output and deterioration in current-voltage characteristics due to shorting of the electrified elements.
It has been proposed to provide separate metal or oxide films over Ag electrodes by such techniques as vapor deposition or sputtering in order to avoid exposed sections on the Ag electrode.
By thus covering the surface of the Ag electrode with a separate metal or oxide film, it is attempted to minimize migration from the Ag electrode (see Japanese Unexamined Patent Publication No. 11-220171, No. 2003-168823 and No. 2005-203618.
Ideally, if the surface of the Ag electrode is completely covered in a protected electrode employing a separate metal it should be possible to prevent deterioration of the element characteristics, but in actual practice it has been difficult to avoid exposure of Ag due to problems in the manufacturing process.
For example, when a different metal is vapor deposited on a wafer having an Ag electrode formed thereon, it has been confirmed that adhesion of particles creates a problem where the metal does not envelop around the areas under the particle shadows, such that when the particles are shed after vapor deposition they leave sections with exposed Ag.
In nitride semiconductor elements, the electrode is generally formed by a technique such as vapor deposition or sputtering. These techniques form films by the kinetic energy produced upon vaporization of metals under high vacuum, but when-the film-forming pressure is between 10−4 Pa and 10 Pa, the mean free path of the metal particles is in the range of several meters to several tenths of millimeters, thereby limiting envelopment of the metal material in the particle shadows.